1. Field of the Invention
The present invention relates to an optical element that generates or detects terahertz waves containing a component in a frequency domain from a millimeter wavelength band to a terahertz wavelength band (30 GHz to 30 THz), an optical device using the optical element, and an apparatus using the optical element. The present invention more particularly relates to an optical element that generates or detects electromagnetic wave pulses containing a Fourier component in the terahertz frequency band through optical pulse irradiation, an optical device using the optical element, and a terahertz time-domain spectroscopic apparatus (THz-TDS apparatus) using the optical element.
2. Description of the Related Art
In recent years, a nondestructive imaging technique using terahertz waves has been developed. The application of electromagnetic waves in the terahertz frequency band has proven to be a powerful technique that provides a perspective imaging apparatus much safer than an X-ray apparatus for performing imaging because terahertz waves have several advantages over x-rays. For example, terahertz waves have very low photon energies (e.g., 4 meV at 1 THz) which are about a million times lower than typical x-ray photon energies (e.g., in the range of keV). Therefore, terahertz waves do not subject a biological tissue to harmful radiation. Also, for example, a spectroscopic technique that determines a physical property of a substance, such as a binding state of molecules, by obtaining an absorption spectrum of the inside of the substance and a complex permittivity; and an analyzing technique for a biological molecule have been developed based on extraordinary noise rejection and extremely high signal-to-noise ratios of terahertz time-domain spectroscopy.
A widely known method for generating terahertz waves is a method of using a photoconductive element. The photoconductive element typically includes a special semiconductor and two electrodes arranged above the semiconductor. This special semiconductor is characterized by a relatively large mobility and a carrier lifetime duration of relatively few picoseconds or shorter. If a gap between the electrodes is irradiated with ultrashort pulsed light while a voltage is applied between the electrodes, excited photocarriers instantaneously carry electric current between the electrodes, and terahertz waves are radiated. The above-mentioned measurement and imaging technique are being studied while such a photoconductive element is also used as a detector for terahertz waves to provide THz-TDS. In the situations of the above techniques, a titanium-sapphire laser is typically used as an excitation source that generates ultrashort pulses of light at a center wavelength of 800 nm. However, to reduce the size and cost, it is desirable to use a fiber laser with a center wavelength in the communication waveband. In this case, the wavelength of the excited light is 1 μm or longer. A low-temperature growth (LT-)GaAs, which has been used for the photoconductive element, behaves as a transparent body for this wavelength. Hence, a GaAs cannot be used. Owing to this, a LT-InGaAs is being studied as a photoconductive material instead of GaAs (see Japanese Patent Laid-Open No. 2006-086227).
InGaAs, however, has a band gap smaller than GaAs, which means that the carrier concentration of an intrinsic semiconductor of this type may increase. Further, a remaining carrier concentration increases as a crystal defect increases. Hence, it is difficult to increase the resistance. Hence, an application voltage cannot be increased, and it is difficult to increase the amount of change in temporal differentiation for the number of photocarriers, as compared with GaAs. Owing to this, terahertz wave generation efficiency has been restricted. The restriction for the generation efficiency of terahertz waves is a considerable bottleneck for the development of the photoconductive element in the communication waveband. Hence, a new photoswitch system has been proposed. The proposed new system uses a Schottky junction without using photoconductivity. See 2008 Conference on Quantum Electronics and Laser Science, Conference on Lasers and Electro-Optics, CLEO/QELS 2008, Article number 4551244. With this system, a basic concept of which has been reproduced in FIG. 6, Schottky junction portions 72 arranged between a semi-insulating InP substrate 70 and Ti/Au electrodes 71 are irradiated with excitation light 73. Accordingly, electrons that obtain energy for crossing a Schottky barrier instantaneously move from the electrodes 71 to the InP substrate 70, and terahertz waves are generated. In this case, the excitation light 73 has a wavelength in a 1.5 μm band. The light is not absorbed by the InP substrate 70, but has a optical energy larger than the height of the Schottky barrier. Since the semi-insulating InP is used, the element has a resistance higher than the InGaAs type, and hence a higher electric field can be applied thereto. Accordingly, terahertz waves can be generated in a highly efficient manner.
However, with the Schottky type element described in 2008 Conference on Quantum Electronics and Laser Science Conference on Lasers and Electro-Optics, CLEO/QELS 2008, Article number 4551244, it is difficult to efficiently irradiate the interface between the electrodes and the semiconductor with the excitation light. This is because, referring to FIG. 6, basically the end portions 72 of the electrodes 71 and the semiconductor 70 are irradiated with the light 73 and only the Schottky junctions formed near the end portions are used as terahertz wave generation regions. That is, since the excitation light 73 is emitted from above the electrodes, the area of Schottky junction regions that are irradiated with the light becomes small, resulting in the generation efficiency of terahertz waves being restricted.